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In Reply to: RE: using cathode bias voltage for preamp tube heaters posted by DAK on July 20, 2014 at 09:36:49
Just outta curiosity, what advantages are there to draw heater current from the cathode bias voltage? Doesn't this put heater noise (albeit DC) into the tube? I would think that one should isolate the cathode from all sources of current draw, outside of the tube's needs.
Follow Ups:
The heater current goes through the bias resistor.. which means you can use a substantially lower value for Rk and avoid using Ck altogether.
Hi!
As I see it just a cost advantage. This is a way to make use of the current which would otherwise be wasted in the cathode resistor.
Thomas
This method was used to good effect in many vintage units with high reliability. Eico, Fisher, and others would take the "wasted" current from power tube bias and instead putting it to work through heating the filaments of the front end tubes. Since it is pure DCV there is no hum induced by AC which is a good thing. regards, Dak
The concept of "waste" is an economic concept.
Done well the heater on a 12AX7 can be very quiet with AC, after all, that is one of the benefits of the indirect heater.
"Done well the heater on a 12AX7 can be very quiet with AC"
AC on a 12AX7 is absolutely NOT quiet enough for a high-end phono preamp. Perhaps the Fisher wasn't properly designed. HK used a circuit that combined resistors with the filament drive circuit, and they also used output tubes operating well under their ratings. These two techniques pretty well ensure that the choice of preamp tubes won't be critical. I can't say it's impossible to damage the outputs, but judging by the schematics, it seems extremely unlikely.
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Harman Kardon also used this method to good end. I've heard a few disparaging remarks about the technique here in the forum, but I don't know why. I've never personally seen it cause a problem. When they're powered by the output bias, the phono tubes don't exhibit that annoying flash at turn-on, and they operate virtually hum free.
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Buy Chinese. Bury freedom.
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